Alertness is essential for sensory processing and efficient responses to environmental changes. While phasic alerting (transient, cue-driven) is known to enhance visual attention, its interaction with tonic alerting (sustained, baseline-reflecting) remains underexplored. In two preregistered experiments, we isolated the effects of phasic and tonic auditory alerting on visual attention using a novel phasic/tonic/no-alerting design and a purely accuracy-based letter recognition task. Computational modeling based on the Theory of Visual Attention (TVA) revealed that phasic alerting significantly enhances visual processing speed without impacting perceptual thresholds, visual short-term memory, or attentional selectivity. Notably, phasic alerting occurred independently of tonic alerting, providing a robust methodological framework for isolating these effects. These findings advance our understanding of the distinct roles of phasic alerting in accelerating early perceptual processing and highlight their implications for attentional dynamics in complex environments. In the final discussion, we will link the approach employed here to recent advances in TVA-based modeling of perceptual effects.